The wasm interpreter was always single-threaded, and there are no plans
to change this. Still, there was a concept of threads, but with the
hard-coded constraint that there is always exactly one of them.

In order to clean up the code, and as a preparation to remove more
unneeded functionality before moving the interpreter over to the test
directory, this CL removes the concept of threads and merges the
{ThreadImpl} class into {WasmInterpreterInternals}.

Drive-by: Remove the dead {GetFrameCount} method.

R=ahaas@chromium.org

Bug: v8:10389
Change-Id: If65cdd21b34ce8debf8ba0f24dbeacec15e0a1d7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2231354Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68204}